layout: Check again for primary monitor after startup animation idle

On some hardware combinations the display can be known, then unknown again. Meaining that when the update monitors function is called it will have a value, then be called again setting this.primaryMonitor to null. If the timing is just right gnome shell will loadBackground, then by the time the animation is ready the monitor will be gone, thus methods will be called on a null value. This adds more checks for a valid primary monitor, and wont play the animation until the system is idle AND has a valid priamry monitor.

Fixes: #5003 (closed)